Pre-heat oven to 350F (180C). Line a cookie sheet with parchment paper.
In a mixing bowl whisk together the flour, salt and baking powder, make a well in the middle and add the sugar, butter, egg, zest (if using) and vanilla. Mix with the flat beaters until almost combined, then add the pistachios (or pistachio/chocolate chip mixture). If too dry than add 1-2 tablespoons of milk, a tablespoon at a time.
Move the dough to a flat surface and form into a log, about 2 ½ inches (6 cm) wide. Place it on the prepared cookie sheet and bake for approximately 20 minutes, remove from the oven, let sit 5-10 minutes then slice into thin slices (¼ -½ inch, I did some of each)place the slices cut side up back on the cookie sheet and bake again for 15 minutes. Let cool 10 minutes on the cookie sheet then move to a wire rack to cool completely. Enjoy!
If you wish you can substitute the sugar with honey, 1 for 1 substitution, just add 2 tablespoons extra of flour.I made one recipe with just pistachios and zest and one with no zest but 1/2 pistachios and 1/2 white chocolate chips. We preferred the one without the zest. Although both were very tasty.